exten => _NXX.,1,SetVar(areacode=${EXTEN:0:3}) - get the first 3 digits of ${EXTEN}
exten => _516XXXXXXX,1,Dial(${EXTEN:3}) - get all but the first 3 digits of ${EXTEN}
exten => 100,1,SetVar(whichVowel=4)
exten => 100,2,SetVar(foo=AEIOU:${whichVowel}:1) - sets ${foo} to the single letter 'U'